phosphoric acid

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Structural formula
Structural formula of ortho-phosphoric acid
Surname phosphoric acid
other names
  • ortho phosphoric acid
  • Phosphoric acid
  • E 338
Molecular formula H 3 PO 4
Brief description

colorless and odorless solid or liquid

External identifiers / databases
CAS number 7664-38-2
EC number 231-633-2
ECHA InfoCard 100,028,758
PubChem 1004
Wikidata Q184782
Molar mass 98.00 g mol −1
Physical state

solid (often liquid, since a supercooled melt can be stored for a long time)


1.87 g cm −3 (25 ° C)

Melting point

42.35 ° C (anhydrous)

boiling point

213 ° C (decomposition, dehydration)

Vapor pressure

3.8 Pa (20 ° C)

pK s value
  • pK s 1 : 2.16
  • pK s 2 : 7.21
  • pK s 3 : 12.32

completely miscible with water

safety instructions
GHS hazard labeling from  Regulation (EC) No. 1272/2008 (CLP) , expanded if necessary
05 - Corrosive


H and P phrases H: 290-314
P: 280-301 + 330 + 331-305 + 351 + 338-308 + 310
  • DFG : 2 mg m −3 (inhalable aerosol fraction)
  • Switzerland: 1 mg m −3
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

The phosphoric or orthophosphoric acid is the main oxygen acid of phosphorus and one of the most important inorganic acids. It is a tri-protonic acid and reacts as a moderately strong acid with regard to the first deprotonation . Their salts and esters are called phosphates , the term organophosphate is also common for esters of phosphoric acid. The phosphorus in these compounds has the oxidation state V.

Thomas Graham did important work to clarify the structure .

The condensates diphosphoric acid , meta- and polyphosphoric acids are also derived from phosphoric acid .

As a food additive , phosphoric acid is declared as E 338 .

Extraction and presentation

Phosphoric acid can be produced from rock phosphate (mostly apatite , Ca 5 (PO 4 ) 3 X with X = F, OH or Cl) and sulfuric acid , hydrochloric acid or nitric acid . As by-products thereby fall CaSO 4 ( phosphogypsum , contaminated gypsum ) and H 2 [SiF 6 ] ( hexafluorosilicic acid ) to. It is the water added and the gypsum is by filtration removed together with other insoluble materials. Fluoride as hexafluoridosilicic acid is removed in a further stage by evaporation .

Although the reaction takes place in stages with calcium dihydrogen phosphate, the overall reaction can be represented as follows:

However, there are side reactions , for example with calcium fluoride and calcium carbonate in the rock :

Fluorosilicic acid is an important by-product in the production of hydrogen fluoride . It can be neutralized with sodium hydroxide to form sodium hexafluorosilicate . The acid is also used to make aluminum fluoride , which in turn is used to make aluminum .

Alternatively, phosphoric acid can be produced by burning elemental phosphorus to phosphorus pentoxide , P 4 O 10 , and subsequent hydrolysis (so-called thermal phosphoric acid). A highly pure product is obtained by concentrating a commercial phosphoric acid solution to a content of over 90 percent and subsequent crystallization in a precisely maintained temperature range.

The raw materials for this process are phosphorus and oxygen :

First, phosphorus is sprayed into the furnace and burned in the air at over 1500 ° C. Most processes use undried air and many require the addition of steam to the phosphorus burner in order to create and maintain a film of condensed polyphosphoric acids that protects the stainless steel burner tower . The products from the burner tower go straight to a hydration tower where the gaseous phosphorus oxide is absorbed in recycled phosphoric acid :

Alternatively, the phosphor can be burned in dried air . The phosphorus pentoxide is condensed as a white powder and separately hydrated to phosphoric acid . This method allows the recovery and reuse of heat .

By Urban mining is phosphoric acid from the leaves of sewage sludge to recover.


The anhydrous substance is very hygroscopic . It is usually sold as an 83-90 % aqueous solution .

The phosphoric acid is a triple proton acid, their protons in three stages of water molecules to form hydronium (H 3 O + ) and to form dihydrogen phosphate -, hydrogen phosphate - or phosphate - anions , can deliver. The associated p K s values are p K s 1 = 2.161; p K s 2 = 7.207 and p K s 3 = 12.325.

Dissociation diagram

Phosphoric acid is a deliquescent solid that generally appears as a colorless, viscous aqueous solution . It is weakly acidic with three possible successive deprotonation steps that form phosphates . Like carboxylic acids, it can dimerize to phosphoanhydrides via a dehydration reaction . It is called tribasic because it has three possible dissociation steps .

One of the most important reactions of phosphoric acid and its derivatives is multimerization. As with carboxylic acids , two phosphoric acid molecules can combine with loss of water to form a diphosphate ester , also known as pyrophosphate .


The rust-brown areas are converted into a matt shimmering iron compound by phosphoric acid.
Phosphoric acid is used for etching of wafers used.

Phosphoric acid is used as a raw material for the production of fertilizers containing phosphate , detergents , rust removers and rust converters as well as for the passivation of iron and zinc to protect against corrosion .

It acts as a rust converter when applied directly to rusty iron , steel and other metal surfaces . It converts red-brown iron, i.e. iron (III) oxide , into black iron (III) phosphate . After this treatment, the black iron phosphate coating can be easily washed off, revealing the fresh metal surface underneath.

It is used to prepare buffer solutions (see phosphate buffer ).

Phosphoric acid is corrosive in high concentrations ; diluted it is used in the food industry as a preservative , acidifier , acid regulator and antioxidant (to prevent fats from becoming rancid and discolouring, for example, meat, sausage or cake fillings) (E 338). Except for its caustic effect, phosphoric acid is non-toxic to the human organism.

It is used as an acidifier for foods and beverages , such as cola . However, this particular use of phosphoric acid has created controversy that has raised many questions about its possible health effects on the human body .

In dentistry , phosphoric acid is combined with zinc powder to form zinc phosphate , which is used as a temporary dental cement . It is also used in orthodontics as a caustic solution to clean and roughen the surface of teeth before inserting brackets and other dental equipment. It is also used in many tooth whitening solutions to remove dental plaque that may be present on the surface of the teeth.

It is used as an electrolyte in phosphoric acid fuel cells . It is also used as a cleaner in the construction industry to remove mineral deposits, cement smears and hard water.

Hot phosphoric acid is in the microelectronics for the etching of silicon nitride is used. It is used by hobbyists as a flux to aid the soldering process . It is also in hydroponics -Ph- solutions used to the pH of nutrient solutions to reduce.

Phosphoric acid is also used as an electrolyte in the electropolishing of copper for deburring . It is commonly used as a wet etchant when processing compound semiconductors .

Biological importance

Phosphate is found in DNA . The phosphorus atoms are marked in yellow.
Spatial structure of adenosine triphosphate

Phosphoric acid esters and polyphosphates play a central role in the metabolism, especially as energy and group carriers (see e.g. ATP and GTP ). Among other things, they are an integral part of DNA , RNA and many coenzymes .

In medicine , fortifying food with phosphoric acid (E 338) is contraindicated in certain clinical pictures. Diseases such as chronic kidney failure (also with dialysis treatment ), osteoporosis and urolithiasis (calcium phosphate stones) require a low-phosphate diet . (see also main article hyperphosphataemia )

Due to its cycle in the environment and in the body, the phosphate content does not change significantly. When phosphoric acid is dissolved in water at neutral pH , the solution mainly contains the dihydrogen phosphate anion and the hydrogen phosphate dianion . However, when incorporated into various biological functions, it is usually in the form of the phosphate ion itself, devoid of hydrogen atoms and three negative charges . Together with calcium ions, it forms hydroxyapatite in the body , which is made up of bones and teeth . The inorganic part of the bones consists of around 40 percent calcium and almost 60 percent phosphate .

The acid in DNA is actually phosphoric acid, but it has replaced two protons with carbon atoms from two different sugars , the ribose part. It is the backbone of DNA that holds the long chain together with covalent bonds , but does not transmit any genetic information.

Adenosine triphosphate consists of adenine , ribose and three phosphates . This molecule is the universal energy currency of the biological world. When it reacts with water and loses a phosphate ion ( hydrolysis ), energy is released. Due to the complex mechanisms in our enzymes , this energy can be used to drive countless processes in all living things . Most fats , sugars, and carbohydrates are used in building adenosine triphosphate when not stored in one form or another.


The phosphoric acid and soluble phosphates can be detected by precipitation with ammonium heptamolybdate to yellow ammonium molybdate phosphate , or by precipitation with magnesium ions in ammoniacal solution to form MgNH 4 PO 4 .

See also


Web links

Commons : Phosphoric Acid  - Collection of Pictures, Videos and Audio Files
Wiktionary: Phosphoric acid  - explanations of meanings, word origins, synonyms, translations
  • Safety data sheet

Individual evidence

  1. Entry on PHOSPHORIC ACID in the CosIng database of the EU Commission, accessed on February 26, 2020.
  2. a b c d e f g h i Entry on phosphoric acid in the GESTIS substance database of the IFA , accessed on January 8, 2020(JavaScript required) .
  3. a b c d e f Entry on phosphoric acid. In: Römpp Online . Georg Thieme Verlag, accessed on May 29, 2014.
  4. Entry on Orthophosphoric acid in the Classification and Labeling Inventory of the European Chemicals Agency (ECHA), accessed on February 1, 2016. Manufacturers or distributors can expand the harmonized classification and labeling .
  5. Swiss Accident Insurance Fund (Suva): Limit values ​​- current MAK and BAT values (search for 7664-38-2 or phosphoric acid ), accessed on November 2, 2015.
  6. ^ A b University of York Center for Industry Education Collaboration: Phosphoric acid
  7. Georg Brauer (Ed.), With the collaboration of Marianne Baudler a . a .: Handbook of Preparative Inorganic Chemistry. 3rd, revised edition. Volume I, Ferdinand Enke, Stuttgart 1975, ISBN 3-432-02328-6 , pp. 528-529.
  8. Media release from the building department of the Canton of Zurich: Converting sewage sludge into raw material: New process suitable for industrial production. June 3, 2019, accessed October 14, 2019 .
  9. BYJU'S: Phosphoric Acid
  10. P. Kurzweil u. a .: Chemistry: Basics, advanced knowledge, applications and experiments. Springer, 2012, p. 151, ISBN 3-8348-1555-1 , limited preview in the Google book search.
  11. JF Diehl: Chemistry in food: residues, impurities, ingredients and additives. John Wiley & Sons, 2008, p. 18, ISBN 3-527-62461-9 , limited preview in Google Book Search.
  12. ^ ScienceStruck: Uses of Phosphoric Acid
  13. E. Lückerath et al. a .: Dietetics and Nutritional Advice: The Practice Book. Georg Thieme Verlag, 2011, p. 272, ISBN 3-8304-7563-2 , limited preview in the Google book search.
  14. R. Nowack et al. a .: Dialysis and nephrology for nursing professions. Springer, 2002, p. 292, ISBN 3-540-42811-9 , limited preview in the Google book search.
  15. ^ Chemistry World: Phosphoric Acid